Similar evolutionary paths in the domestication of goats and sheep

By comparing the genome changes observed in goat and sheep, researchers has taken the first step towards characterising the genetic basis underpinning the ‘domestication syndrome’.

During domestication processes, humans have shaped the characteristics of wild species in order to exploit them. The genetic mechanisms behind induced changes are still poorly understood to this day, including those that would explain the occurrence of similar traits in different domestic species. However, by comparing the genome changes observed during two independent domestication processes (goat and sheep), a European consortium led by French researchers from the University of Grenoble Alpes and the CNRS (Laboratory of Alpine Ecology - Univ. Grenoble Alpes, Univ. Savoie Mont-Blanc, CNRS), working in conjunction with (CEA) [1], has taken the first step towards characterising the genetic basis underpinning such changes that make up what we refer to as the ‘domestication syndrome’. This study was published in the March 6, 2018 issue of Nature Communications.

Domestication is a key turning point in the evolution of mankind that led to the emergence of agriculture during the Neolithic period. Domestic animals differ from their wild counterparts in terms of the often similar morphological, behavioural and physiological changes that they have undergone. During the initial steps of domestication, the selection made by humans for tameness or changes in development rates is said to have caused the unintended emergence of specific traits, such as the appearance of specific colour patterns (piebald), size changes and shift in reproductive timing. Subsequently, a deliberate selection allowed for productivity improvements (meat, milk, etc.).

Several recent studies have identified genes, the variation in which distinguishes domestic species ch as the chicken, pig, rabbit or horse from their wild counterparts. However, the existence of genetic bases common to different domestication processes had hitherto been unexplored.

To study this phenomenon, the Nextgen consortium led by a team from the Laboratory of Alpine Ecology used the parallel domestication histories of the goat and sheep. Their wild ancestors, the Bezoar ibex and the Asiatic mouflon, were domesticated around 10,500 years ago in the same area of the Middle East, where they still live to this day (Eastern Turkey and Western Iran). Since then, human beings have spread goats and sheep across the world beyond their natural distribution area, which enables us to study the genome changes linked to domestication in different environments and breeding systems.

The genomes of over 140 wild and domestic individuals from local, traditional and improved breeds were thus sequenced by researchers. A comparison of the DNA sequences has enabled us to identify, for each species pair (goat/ibex and sheep/mouflon), around forty regions where the domestic genomes differ from the wild genomes. These genomic regions contain genes that are involved in the functioning of the nervous system or immune responses, or are linked to breeding traits (coat, meat, milk, reproduction).

The major discovery is that 20 of these regions are common to goats and to sheep, but that for several of them the mechanisms underlying the differentiation between wilds and domestics differ according to the species.

This outcome calls into question a simplistic vision of the effects of domestication on genomes based on uniform selective processes targeting relatively canalized genetic networks. It suggests that, even though the same genomic regions may have played a key role in independent domestication processes, the precise mechanisms underpinning similar morphological or physiological changes may have differed.

This study is a first step towards understanding the genetic mechanisms responsible for the domestication syndrome. Future research should allow us to pinpoint, for a broader set of species, the genetic targets of the domestication syndrome, and to identify the related evolutionary scenarios, for example by distinguishing the regions selected without forethought - during the early steps of domestication from those selected later on to intentionally improve certain traits of interest.

(1) The European consortium that conducted this study is headed up by the Laboratory of Alpine Ecology ((LECA – Univ. Grenoble Alpes / CNRS / Univ. Savoie Mont-Blanc), working in conjunction with bodies such as the CEA (Institut François Jacob/Genoscope)